1. Field of the Invention
The present invention relates to a vertical injection apparatus incorporated with a mold defining a cavity into which a melt is injected from just below the cavity, and more particularly, to a cavity designed for a mold product such as a disk wheel having a central hole.
2. Description of the Related Art
The casting of an automobile disk wheel of aluminum is often performed by a vertical die casting machine because an inclusion of gas at the melt-injecting step is thus reduced. FIG. 6 (prior art) is a schematic diagram illustrating the longitudinal section of a mold and an injection apparatus in a conventional die casting machine of this type. This conventional machine will now be described with reference to FIG. 6. A lower mold half 112 having a cylindrical convex part at the center is attached to a stationary platen 111 secured on a machine base, and an upper mold half 114 having a low convex part at the center is attached to a vertically movable platen 113 supported on a moldclamping cylinder (not shown). A plurality of cores 115 are inserted between both the molds halves 112 and 114 from a plurality of positions in the circumferential direction so that the cores 115 can move in the horizontal direction in accordance with the advance and retreat of a piston rod 117 of a cylinder 116 supported on the side of the movable platen 113. A cavity 118 is defined by both the mold halves 112 and 114 and the plurality of cores 115. An injection sleeve 119 is freely extractably inserted from below into a sleeve hole formed in the stationary platen 111 and the lower mold half 112, and a plunger tip 120 is fitted in the injection sleeve 119 so that the plunger tip 120 can be advanced and retreated by an injection cylinder (not shown). A melt 121 is cast in the state where the injection sleeve 119 is extracted from the sleeve hole.
By adopting the above-mentioned structure, if the melt 121 is cast in the injection sleeve and the plunger tip 120 is inserted into the sleeve hole and then advanced, the melt is injected into the cavity 118, and after the melt 121 is solidified and cooled, the movable platen 113 is raised and the molds are opened. Simultaneously, the cores 115 are opened sideways and a product solidified in the cavity is pushed out and withdrawn from the machine by a product push-out apparatus (not shown).
For explaining the flow of the melt 121 in the cavity 118 at the injection operation, a disk portion 118a of the cavity 118 corresponding to a disk of a disk wheel in FIG. 6 has diagrammatically a disk-like shape as shown in FIG. 7, attached hereto and a rim portion 118b of the cavity 118 has diagrammatically a cylindrical shape as shown in FIG. 8, attached hereto. The melt 121 raised by the plunger tip 120 flows radially in the disk portion 118a as indicated by an arrow in FIG. 7 and drops down under its own weight in the rim portion 8b as indicated by an arrow in FIG. 8. FIG. 9 attached hereto is a perspective view showing the state of the melt flowing in this manner. While the melt 121 thus flowing drops down in the rim portion 118b, coarse and dense portions are formed in the melt flow because of a temperature unevenness in the mold halves 112 and 114, an adhesion unevenness of a parting agent, and scratches on the surfaces of the mold halves 112 and 114. Gas as indicated by reference numeral 122 in FIG. 9 is sometimes included in the melt 121. If filling is completed in this state, voids are formed in the molded article by the gas included in the melt 121.
If injection is carried out in the state where the cavity 118 is arranged so that the disk portion 118a is located above, gas is often included in the melt, as indicated above. This disadvantage may be eliminated if the cavity 118 is arranged so that the rim portion 118b rises when the disk portion 118a is located below. However, if this method is adopted, a hub decorated surface of the product is located on the side of the sleeve 119, and an unnecessary melt-solidified part, called a "biscuit", is formed on this surface. If this part is cut off after molding, the appearance of the decorated surface is degraded. Therefore, according to the conventional technique, molding is always carried out in the state where the disk portion 118a is located above.
According to the conventional technique, molding is carried out in the state where the disk portion 118a is located above, as pointed out hereinbefore. Therefore, in order to avoid an inclusion of gas in the melt, the injection must be conducted at a relatively low speed, and thus the productivity is reduced. Inherently, in order to stabilize the quality of the product, the flow manner of the melt 121 in the cavity 118 should be controlled by the speed of the plunger tip 120. However, for the above-mentioned reason, this control is impossible, and the quality cannot be stabilized. If the control is performed by the speed of the plunger tip 120, the injection speed is elevated and the inclusion of gas is increased.
Where a disk wheel of aluminum is prepared, for example, by such a vertical die casting machine as the above, a gas vent device for a mold is generally used and an annular or circumferential runner communicating with a mold cavity through a plurality of radial gates is arranged between this gas vent device and the mold cavity. When a melt is injected and filled in the cavity, the gas in the cavity and a part of the melt are advanced to the gas vent device through the gates, runner and gas vent passage, and after the gas alone is discharged through a gas vent valve, the gas vent valve is closed by the force of inertia of the melt and the like. When the melt is then coagulated and solidified, the mold is opened and a molded product solidified in the cavity is pushed out to the outside of the cavity by a product push-out apparatus. At this point, a melt-solidified product is formed within the gate, annular runner and gas vent passage, and this melt-solidified product is pushed out simultaneously with the molded product.
In this conventional injection molding apparatus, since the melt-solidified part formed between the annular runner and gates is pushed out simultaneously and integrally with the molded product, the withdrawn molded product must be separated from the melt-solidified part by a hammer or the like. This operation is difficult and reduces the productivity. Furthermore, there is a risk of damage to a part of the product at the separating step. Moreover, at the push-out operation, if the melt-solidified part in the annular runner is moved to the central part of the periphery of the molded product by a cutting separation of the melt-solidified gate part, it is difficult to separate and withdraw the solidified melt runner part from the molded product, and the molded product is often damaged.